高一物理课件：牛顿第二定律(英文版).ppt

Newtonsfirstlaw,Oneday,StephenSuwasdrivingamotorcycleonthestreet.Meanwhile,DavidSunwasdrivingatrunkatafastspeed.StephenSudidnotstopintimeassoonasthetrafficlightturnedred.DavidsuncouldnotcontrolthevanandcollidedwithStephen.Theremainingquestionis,eventhoughStephenslammedonthebrake,theaccidenthappenedanyway.Whatcausedthistragedy?,case,study,tendencyresistinertiamotionabsence,wordlist,AccordingtoNewtonsfirstlaw,anobjectinmotioncontinuesinmotionwiththesamespeedandinthesamedirectionunlessacteduponbyanunbalancedforce.,inertia,Definition,Newtonsfirstlawofmotion牛顿第一定律:Everybodycontinuesinitsstateofuniformmotionorrestunlessacteduponbyanexternalforce,ApplicationoftheNewtonsfirstlaw,SafetybeltsAnypassengersinthecarwillalsobedecceleratedtorestiftheyarestrappedtothecarbyseatbelts.Beingstrappedtightlytothecar,thepassengerssharethesamestateofmotionasthecar.Asthecaraccelerates,thepassengersacceleratewithit;asthecardecelerates,thepassengersdeceleratewithit;andasthecarmaintainsaconstantspeed,thepassengersmaintainaconstantspeedaswell.,practice,Iftwoobjects,auniformdiskandauniformsphere,havethesamemomentofinertiaabouttheiraxesofrotationandthesameangularvelocity,thenthediskhasthelargerrotationalkineticenergy.(T)True(F)False,Iftwoendsofaropearepulledwithforcesofequalmagnitudeandoppositedirection,thetensionatthecenteroftheropemustbezero,TrueFalse,AdiskandahoopofthesamemassMandradiusRrollwithoutslippingacrossahorizontalfloor.Boththediskandthehooparemovingwithvelocityvwhenthefloorstartstoslopeupward.Whichstatementiscorrect?(B)(a)Thediscmakesittoagreaterheightthanthehoop.(b)Thehoopmakesittoagreaterheightthanthedisk.(c)Boththediskandthehoopmakeittothesameheight.,NewtonsSecondLaw,NewtonsSecondLawofmotionpertainstothebehaviorofobjectsforwhichallexistingforcesarenotbalanced.Thesecondlawstatesthattheaccelerationofanobjectisdependentupontwovariables-thenetforceactingupontheobjectandthemassoftheobject.Theaccelerationofanobjectdependsdirectlyuponthenetforceactingupontheobject,andinverselyuponthemassoftheobject.,NewtonsSecondLawtellsusthatrateofincrease(ordecrease)inthespeedofsomethingwhichismovingisproportionaltotheforceactingonit.Imaginethatyouareridingabicyclealongaperfectlysmoothandlevelroadandyoudecidetostoppedaling.Ifthereisastrongwindpushingagainstyou,youwillstopcompletelyinashortertimethanyouwouldifthewindwaslight.,Applications,NowthatweknowNewtonsLawsofMotion,howdoweapplythem?Howcantheyletuspredictthemotionofanobjectifweknowalltheforcesactinguponit?Howcantheyletuspredicttheforcesonanobjectifweknowitsmotion?,FORCE=MASStimesACCELERATION,or,Kobescar,whichweighs1,000kg,isoutofgas.Kobeistryingtopushthecartoagasstation,andhemakesthecargo0.05m/s2.UsingNewtonsSecondLaw,youcancomputehowmuchforceKobeisapplyingtothecar.,Answer=50newtons,Example1:Consideracratebeingpulledalongahorizontal,frictionlessfloor.AropeistiedarounditandamanpullsontheropewithaforceofT.Tisthetensionintherope.Whathappenstothecrate?,Theseforcesareshownonthefreebodydiagramabove.Wehavedrawninalltheforcesactingontheobject.Thenetforceisthevectorsumoftheseforces.,Fnet=F=T+n+w,Fnet,x=Fx=Tx+nx+wx,Fnet,y=Fy=Ty+ny+wy,n=w,Example2:Thisparticularexampleisstatedintermsofweighingafishinanacceleratingelevator.Itisalsofuntothinkofweighingyourselfinanacceleratingelevator.Whendoesanelevatoraccelerateupwards?Whendoesanelevatoracceleratedownwards?,Theforcesactingonthefishareshowninthefree-bodydiagram.Tisthetensionsuppliedbythescale.Thisisthevaluethescalereads.Wemaycallittheapparentweightofthefish.Thenetforceonthefishis,Fnet=T-mg,Thenetforceis(always!)equaltothemasstimestheacceleration.Thisfishismovingalongwiththeelevator.Inthisdiagramwehavetakentheaccelerationtobeupsoitispositive.,Fnet=T-mg=maT=mg+maT=m(g+a),Whiletheelevatoracceleratesupward,theapparentweightofthefishisgreaterthanitstrueweight,mg.,Theforcesonthefishareagainshowninthefree-bodydiagram,Fnet=T-mg,Thenetforceis(always!)equaltothemasstimestheacceleration.Thisfishismovingalongwiththeelevator.Nowtheaccelerationtobeupsoitisnegative.,Fnet=T-mg=m(-a)T=mg-maT=m(g-a),Whiletheelevatoracceleratesdownward,theapparentweightofthefishislessthanitstrueweight,mg.,1.WhowasthescientistwhogaveustheLawsofMotion?Answer:SirIsaacNewton2.HowmanyLawsofMotionarethere?Answer:three3.Whatisanothernameforthefirstlawofmotion?Answer:LawofInertia4.Whichlawexplainswhyweneedtowearseatbelts?Answer:FirstLawofMotion,5.Whichlawsaysthatforceisequaltomasstimesacceleration(F=MA)?Answer:SecondLawofMotion6.Whichlawsaysthatheavierobjectsrequiremoreforcethanlighterobjectstomoveoraccelera